Field of the Invention
The present invention concerns a method to acquire magnetic resonance data, as well as a magnetic resonance system, and an electronically readable data storage medium.
Description of the Prior Art
Magnetic resonance (MR) is a known modality with which images of the inside of an examination subject can be generated. Expressed in a simplified manner, for this purpose the examination subject, in the opening of a magnetic resonance apparatus, is positioned in a strong, static, homogeneous basic magnetic field (also called a B0 field) with a field strength of 0.2 to 7 Tesla or more, such that nuclear spins in the subject orient preferentially along the basic magnetic field. Radio-frequency (RF) excitation pulses and possibly refocusing pulses are radiated into the examination subject to elicit magnetic resonance signals, which are detected and entered as data values into an electronic memory, in an organized manner that represents a domain known as k-space, such as a matrix. On the basis of the k-space data, MR images are reconstructed or spectroscopy data are determined. Rapidly switched (activated) magnetic gradient fields may be superimposed on the basic magnetic field for spatial encoding of the magnetic resonance data (measurement data). The acquired measurement data are digitized and stored as complex numerical values in a k-space matrix. For example, by means of a multidimensional Fourier transformation, an associated MR image can be reconstructed from the k-space matrix populated with values.
The aforementioned radio-frequency pulses and gradient fields are activated in the magnetic resonance apparatus according to various schemes, known as pulse sequences, with which the data acquisition unit is operated. Highly sophisticated spin-echo pulse sequences include single-slab three-dimensional (3D) turbo or fast spin-echo (hereafter 3D-TSE/FSE) pulse sequences known as, among other names, SPACE (Sampling Perfection with Application optimized Contrasts using different flip angle Evolutions). Pulse sequences of this type allow an extremely large number of refocusing RF pulses (e.g., more than 300), and may use a refocusing RF pulse train that exhibits pulses with mostly constant flip angles, or that exhibits pulses with respectively different flip angles of less than 180°, throughout the duration of the echo train. In the latter case, the values of the flip angles for the RF pulse train are selected so as to achieve desired signal strengths for different types of tissue, and are referred to as a flip angle evolution. Thus, this implementation of a 3D-TSE/FSE pulse sequence uses application-specific variable flip angles.
Many applications of the 3D-TSE/FSE pulse sequence (3D-TSE/FSE protocol) require a long echo time (TE). For example, magnetic resonance cholangiopancreatography (MRCP) investigations in the abdomen make use of a TE that is often greater than 500 ms and refocusing RF-pulse flip angles that are relatively high (ideally, close to 180° for this application), and at the same time a high loading of the magnetic resonance system and/or a high reflection factor may exist. “High loading” of the magnetic resonance system means that the RF transmitter, which is used to generate the radiated RF pulses, has a high transmitter reference voltage. Under such circumstances, it is often the case that the RF amplifier in the RF transmission system cannot manage to emit a complete RF pulse train, as needed. This problem is sometimes called a “burst error” and is often an inherent factor of the magnetic resonance installation itself. Although the discussion herein relates primarily to the aforementioned 3D-TSE/FSE pulse sequence, the same problem exists with other types of magnetic resonance imaging pulse sequences, such as single-shot two-dimensional turbo or fast spin-echo, or multi-slice two-dimensional turbo or fast spin-echo.
In order to achieve a long TE in the 3D-TSE/FSE pulse sequence, a long RF pulse train is required, in which many refocusing pulses are radiated within a short time. A specific example based on a realistic protocol is, with an echo spacing (ESP) of 4 ms and TE=700 ms, that at least 700/4=175 pulses are needed. Further, some additional echoes after the central echo may be acquired (as may be needed, for example, for a partial Fourier reconstruction). In this example, 183 RF pulses may then be required.
The radiation of so many RF pulses with a high flip angle within a short time unloads (discharges) the capacitors in the RF amplifier. In addition, if the loading is high (as is the case for the aforementioned abdominal examinations), the capacitor's charging and recharging rate is not sufficient so as to allow the necessary echo train to be achieved.
In order to address this problem, a “workaround” is known in which an automatic reduction of the flip angle of the refocusing pulses, down to as low as 100°, is implemented. The flip angle reduction, however, decreases the quality of the images that are reconstructed from the magnetic resonance data obtained in such a manner. Additionally, in many cases the reduction down to 100° is not sufficient, but this flip angle reduction cannot be significantly exceeded while maintaining clinically useful image quality, and represents a basic limit in the operation of the magnetic resonance apparatus. As a consequence, due to the “burst error,” in some instances the 3D-TSE/FSE protocol cannot be employed. In situations where this precludes an MRCP protocol from being used, an appropriate clinical diagnosis may be precluded.